1. Field
This patent specification relates to a method and apparatus for electrophotographic image forming, and more particularly to a method and apparatus for photographic image forming capable of effectively fixing an image with heat and pressure using a roller with a pressure plate.
2. Description of the Related Art
FIG. 5 illustrates a background fixing apparatus included in an image forming apparatus such as a printer, a facsimile apparatus, a copying machine, etc. The background fixing apparatus includes a fixing roller 50, a pressure roller 51 contacting the fixing roller under pressure, a fixing heater 52 mounted inside the fixing roller 50 to apply heat to the fixing roller 50, and a temperature detecting mechanism 53 for detecting a temperature of the fixing roller 50. The background fixing apparatus further includes a separation pawl 54 for separating a recording sheet from the fixing roller 50, a cleaning pad 55 contacting the fixing roller 50 to clean the surface of the cleaning roller 50, and a cleaning pad holder 56 for supporting the cleaning pad 55. The background fixing apparatus further includes a cleaning pad stopper 57 mounted at a position downstream in a rotation direction of the fixing roller 50 relative to the cleaning pad 57, an inlet guide plate 58 for guiding the recording sheet, a sheet ejection roller 59 for ejecting the recording sheet after the fixing process, and a roller 60 contacting the sheet ejection roller 59.
The recording sheet carrying a toner image thereon is guided by the inlet guide plate 58 into a nip portion formed between the fixing roller 50 and the pressure roller 51. The toner image on the recording sheet is subjected to a fixing process with heat and pressure at the nip portion and, after that, the recording sheet is ejected outside by the sheet ejection roller 59. As the recording sheet passes through the nip portion, the cleaning pad 55 cleans the residual toner off the surface of the fixing roller 50.
A surface temperature of the fixing roller 50 is detected by the temperature detecting mechanism 53, and a heating control is managed by a controller (not shown) based on the temperature detected by the temperature detecting mechanism 53.
Conventionally, fixing apparatuses commonly employs an infrared heater or a halogen heater as a fixing heater and a pair of a fixing roller and a pressure roller. In this configuration, the pressure roller has a relatively large thermal capacity due to a structure in which a metal core is covered with a silicone rubber layer on which a releasing layer of a material such as a fluoro resin is coated. Therefore, the temperature of the pressure roller typically cannot be increased rapidly. This can become a cause of a failure of the fixing process. Also, the above-mentioned configuration requires a relatively high component cost. Moreover, this configuration needs an application of a relatively high fixing pressure to form a sufficient nip width between the fixing roller and the pressure roller, stressing the recording sheet in a way that can produce a fixing wrinkle in the recording sheet.
A different fixing apparatus has been proposed to address at least some of the above-mentioned problems. This fixing apparatus uses a sheet-shaped pressure plate that presses the fixing roller to form a nip for fixing a toner image when a recording sheet is passing through the nip. This sheet-shaped pressure plate can be heated quickly and is capable of forming a sufficient nip width, thereby improving a thermal efficiency in comparison to the earlier-described fixing apparatus.
In general, transportation speeds V1, V2, and V3 of the sheet transfer mechanism, the fixing mechanism, and the sheet ejection mechanism, respectively, have relationships expressed as V1 greater than V2 and V3 greater than V2.
In the background fixing apparatus using the pressure roller, a gripping force relative to the recording sheet can easily be obtained because the pressure roller itself is rotated, so that the transportation of the recording sheet would not be adversely affected by a phenomenon in which the recording sheet gripped by the fixing roller and the pressure roller is pulled by the sheet ejection mechanism or pushed by the fixing roller and the pressure roller.
In the background fixing apparatus using the sheet-shaped pressure plate instead of the pressure roller, however, the transportation force of the fixing roller is generally less than those of the transfer roller and the sheet ejection roller and therefore the recording sheet is mainly transferred by the transportation force of the transfer roller when the recording sheet passes through the fixing apparatus. Accordingly, if the above-mentioned relationships associated with the transportation speeds of the rollers are satisfied in the background fixing apparatus using the sheet-shaped pressure plate, that is the relationships V1 greater than V2 and V3 greater than V2 are satisfied, as in the case of the fixing apparatus using the pressure roller, problem can arise if the recording sheet gripped by the fixing roller and the pressure plate is undesirably pulled by the sheet ejection mechanism or pushed by the fixing roller and the pressure roller. In such a case, the transportation of the recording sheet may become unstable at the nip, causing disruptions of the image forming operations.
This patent specification describes a novel image forming apparatus. In one example, a novel image forming apparatus includes a photosensitive member, a transfer mechanism, a fixing mechanism, a sheet ejection mechanism, and a driving source. The photosensitive member is arranged and configured to carry a toner image thereon. The transfer mechanism is arranged and configured to transfer the toner image carried on the photosensitive member onto a recording sheet. The transfer mechanism including a transfer roller. The fixing mechanism is arranged and configured to fix the toner image onto the recording sheet with heat and pressure. The fixing mechanism includes a fixing roller including a cylindrical roller member containing a heater and a pressure plate having a sheet shape, having a property of a leaf spring, and contacting the fixing roller under pressure. Such a fixing mechanism applies heat and pressure to the toner image carried on the recording sheet when the recording sheet is caused to pass through a fixing nip portion formed between the fixing roller and the pressure plate. The sheet ejection mechanism is arranged and configured to eject the recording sheet coming out from the fixing mechanism. The sheet ejection mechanism includes a pair of sheet ejection rollers. The driving source is arranged and configured to drive the photosensitive member, the transfer roller, the fixing roller, and the sheet ejection rollers to transfer the recording sheet at a linear speed in a substantially steady manner from the photosensitive member through to the sheet ejection mechanism.
The driving source may drive the photosensitive member, the transfer roller, the fixing roller, and the sheet ejection rollers to transfer the recording sheet at a substantially constant speed from the photosensitive member through to the sheet ejection mechanism within a range to cover variations of the linear speeds of the fixing roller and the sheet ejection rollers due to a thermal expansion.
The linear speed of the fixing roller may be set to a value based on a diameter of fixing roller at an allowable lowest temperature and the sheet ejection rollers can be made of material having suitable properties of thermal expansion and elasticity.
The linear speed of the sheet ejection rollers may be adjusted in accordance with a thickness of the material having properties of thermal expansion and elasticity.
The recording sheet may be caused to contact at least one of the transfer roller and the sheet ejection rollers during the time the recording sheet passes through the fixing mechanism.
This patent specification further describes a novel image forming method. In one example, a novel image forming method includes the steps of providing and driving. The providing step provides a photosensitive member for carrying a toner image thereon and a transfer mechanism for transferring the toner image carried on the photosensitive member onto a recording sheet. The transfer mechanism includes a transfer roller. The providing step further provides a fixing mechanism for fixing the toner image onto the recording sheet with heat and pressure. The fixing mechanism includes a fixing roller including a cylindrical roller member containing a heater and a pressure plate having a sheet shape, having a property of a leaf spring, and contacting the fixing roller under pressure. The fixing mechanism applies heat and pressure to the toner image carried on the recording sheet when the recording sheet is caused to pass through a fixing nip portion formed between the fixing roller and the pressure plate. The providing step further provides a sheet ejection mechanism for ejecting the recording sheet coming out from the fixing mechanism. The sheet ejection mechanism includes a pair of sheet ejection rollers. The driving step drives the photosensitive member, the transfer roller, the fixing roller, and the sheet ejection rollers to transfer the recording sheet at a linear speed in a substantially steady manner from the photosensitive member through to the sheet ejection mechanism.